 <!DOCTYPE html>
 <html lang="zh-CN">
 <head>
     <meta charset="UTF-8">
     <meta http-equiv="X-UA-Compatible" content="IE=edge">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>Document</title>
     <style>
         /* 把获得光标的表单选出来 */
         input:focus{
             background-color: chartreuse;
         }  
     </style>
 </head>
 <body>
     <input type="text">
     <input type="button" value="测试按钮">
 </body>
 </html>